1. Field of the Invention
The present invention relates to a solid-state image capturing apparatus, a driving method of the solid-state image capturing apparatus, and an electronic information device, and in particular to a solid-state image capturing apparatus for performing a pixel row selecting operation to obtain an image signal with a wide dynamic range, using a short time exposure and a long time exposure, a driving method of the solid-state image capturing apparatus, and an electronic information device having the solid-state image capturing apparatus used therein.
2. Description of the Related Art
While CCD image sensors and CMOS image sensors are conventionally used for image capturing apparatuses, for outputting an image signal by capturing an image of a subject, CMOS image sensors are recently used more often for mobile devices and the like, from the perspective of electrical power saving, the CMOS image sensors consuming less power than CCD image sensors. In such image sensors, the exposure time is adjusted such that the level becomes appropriate for image signals obtained by capturing an image of a subject.
In a case where brightness is widely different depending on different parts of a subject, it is difficult to obtain, by the adjustment of exposure time, a captured image without deterioration of resolution between a low brightness portion and a high brightness portion of the subject. That is, the signals of the low brightness portion will be destroyed and the resolution of the low brightness portion will not be obtained if the exposure time is adjusted such that the signal level becomes fitting for a high brightness portion of the subject. At the same time, image signals which are read from an image capturing element also become weak, resulting in the poor S/N ratio. On the contrary, the signals of the high brightness portion will be saturated and the image of that portion will only be white in color thereby making it difficult to distinguish the subject, if the exposure time is adjusted such that the signal level becomes appropriate for a low brightness portion.
Therefore, Reference 1 discloses a way to obtain a captured image with a wide dynamic range in CMOS image sensors by performing both a long time exposure and a short time exposure, storing image data obtained by one of the exposures into a memory, reading the image data in the memory when the image data obtained by the other exposure is read out, and mixing both of the image data.
Further, Reference 2 discloses a way to obtain a captured image with a wide dynamic range in CMOS optical sensors, by dividing one frame period into three, and by using both image signals, the image signals with short accumulation time (photoelectric conversion time) which are obtained in one of two periods among the three periods, and the image signals with long accumulation time which are obtained in the other of the two periods. Hereinafter, Reference 2 will be briefly described.
FIG. 20 is a diagram illustrating the CMOS optical sensor disclosed in Reference 2 described above.
A CMOS optical sensor 10 includes: a pixel section 1 in which pixels are arranged in matrix; a vertical selection section 7 for selecting a pixel row in the pixel section 1; a first sample hold CDS section 10a for sample holding a reset signal and a data signal from each pixel in a selected pixel row to generate a difference signal; and a second sample hold CDS section 10b for sample holding a reset signal and a data signal from each pixel in the selected pixel row at a timing different from the CDS section 10a to generate a difference signal. The CMOS optical sensor 10 includes: a first horizontal selection section 12a for successively outputting the difference signal generated in the first sample hold CDS section 10a to a horizontal signal line 14a as, for example, a pixel signal with a long electric charge accumulation time (long time accumulation signal); and a second horizontal selection section 12b for successively outputting the difference signal generated in the second sample hold CDS section 10b to a horizontal signal line 14b as, for example, a pixel signal with a short electric charge accumulation time (short time accumulation signal). The CMOS optical sensor 10 includes: a first output section 16a for amplifying the long time accumulation signal, which is the difference signal outputted to the horizontal signal line 14a; a second output section 16b for amplifying the short time accumulation signal, which is the difference signal outputted to the horizontal signal line 14b; and a timing generator 6 for supplying a timing pulse to the vertical selection section 7, first and second horizontal selection sections 12a and 12b, and first and second sample hold CDS sections 10a and 10b. Furthermore, the pixel section 1 includes a vertical signal line provided (not shown) for reading a reset signal and a data signal from each of the pixels for each pixel column. The vertical signal line is supplied with a constant current by a constant current section 5.
In the CMOS optical sensor 10 with the configuration described above, the vertical selection section 7 and the first and second sample hold CDS section 10a and 10b are controlled by the timing pulse from the timing generator 6 such that two kinds of data signals having different accumulation times, or the long time accumulation signal and the short time accumulation signal, from each pixel row in the pixel section 1 are sample-held by corresponding sample hold CDS section 10a and 10b. Herein, for example, the long time accumulation signal of each pixel in a selected pixel row is supplied to the first sample hold CDS section 10a, and the short time accumulation signal of each pixel in the selected pixel row is supplied to the second sample hold CDS section 10b. As a result, the first and second sample hold CDS section 10a and 10b respectively maintain the long time accumulation signal and the short time accumulation signal for the same pixel.
The long time accumulation signal maintained in the first sample hold CDS section 10a is successively read out to the horizontal signal line 14a by the first horizontal selection section 12a and is supplied to the first output section 16a. In the first output section 16a, the long time accumulation signal is amplified and AD converted to be outputted as a digital pixel value having a long accumulation time. Similarly, the short time accumulation signal maintained in the second sample hold CDS section 10b is successively read out to the horizontal signal line 14b by the second horizontal selection section 12b and is supplied to the second output section 16b. In the second output section 16b, the short time accumulation signal is amplified and AD converted to be outputted as a digital pixel value having a short accumulation time.
By combining the digital pixel value having a long accumulation time and the digital pixel value having a short accumulation time, it becomes possible to obtain an image with a wide dynamic range.
Reference 1: Japanese Laid-Open Publication No. 2003-169259
Reference 2: Japanese Laid-Open Publication No. 2006-115547